1. Field of the Invention
The present invention relates generally to digital signal transmission, and more particularly, to a system and method for multiplexing a plurality of digital services, including compressed imaging services and ancillary data services such as closed captioning for the deaf associated with the imaging services for transmission to a plurality of remote locations.
2. Description of Related Art
The background of the present invention is described herein in the context of pay television systems, such as cable television and direct broadcast satellite (DBS) systems, that distribute a variety of program services to subscribers, but the invention is by no means limited thereto except as expressly set forth in the accompanying claims.
In the pay television industry, programmers produce programs for distribution to various remote locations. A "program" may consist of video, audio and other related services, such as closed-captioning and teletext services. A single programmer may wish to supply many programs and services. Typically, a programmer will supply these services via satellite to individual subscribers (i.e., DBS subscribers) and/or cable television operators. In the case of cable television operators, the services transmitted via satellite are received at the operator's cable head-end installations. A cable operator typically receives programs and other services from many programmers and then selects the programs/services it wishes to distribute to its subscribers. In addition, a cable operator may insert locally produced services at the cable-head end. The selected services and locally produced services are then transmitted to the individual subscribers via a coaxial cable distribution network. In the case of DBS subscribers, each subscriber is capable of receiving a satellite down-link from the programmers directly.
In the past, pay television systems, including cable and DBS systems, have operated in the analog domain. Recently, however, the pay television industry has begun to move toward all digital systems wherein, prior to transmission, all analog signals are converted to digital signals. Digital signal transmission offers the advantage that digital data can be processed at both the transmission and reception ends to improve picture quality. Further, digital data compression techniques have been developed that achieve high signal compression ratios. Digital compression allows a larger number of individual services to be transmitted within a fixed bandwidth. Bandwidth limitations are imposed by both satellite transponders and coaxial cable distribution networks, and therefore digital compression is extremely advantageous.
Further background can be found in U.S. patent application Ser. No. 968,846, filed Oct. 30, 1992, titled System and Method For Transmitting a Plurality of Digital Services. This application is hereby incorporated by reference as if fully set forth herein.
With the growing trend toward a merger of the previously separate technologies of telecommunications including voice and data telecommunications and television including satellite, broadcast and cable television, there has emerged an increased interest in developing adaptable transmission systems capable of handling any one or more of a collection or plurality of such services. The primary media investigated for providing such services to date comprise, for example, coaxial cable, land-based microwave, so-called cellular radio, broadcast FM, broadcast satellite and optical fiber, to name a few.
Each media has its own characteristics. For example, comparing cable and satellite for digital data transmission, cable tends to have a medium error rate, but, when errors appear, the errors come in long bursts. Satellite as a media has a pretty poor error rate, primarily due to the requisite weak signal power, and hence, low signal to noise ratio. In satellite, then, the poor error rate is specially corrected utilizing such techniques as convolutional error correctors, not required in a cable environment.
In copending U.S. application Ser. No. 07/968,846 filed Oct. 30, 1992 and entitled "System and Method for Transmitting a Plurality of Digital Services", the disclosure of which is incorporated herein by reference, there is described an encoder for generating a multiplexer data stream carrying services to remote locations via, for example, a satellite or a cable distribution network. The generated data stream comprises a continuous sequence of frames, each frame comprising two fields, and each field comprising a plurality of lines. A first group of lines of a field defines a transport layer and a second group of lines defines a service data region. A feature of the disclosed scheme is the ability to dynamically vary the multiplexed data stream from field to field. A further feature of the disclosed scheme is that the data transmission rate of the multiplexed data stream is related to the frequency of known analog video formats, i.e. frame, field and horizontal line rates.
In copending U.S. application Ser. No. 07/970,918 filed Nov. 2, 1992, entitled "System and Method for Multiplexing a Plurality of Digital Program Services for Transmission to Remote Locations", the disclosure of which is incorporated herein by reference there is described another system, this for multiplexing a plurality of digital program services comprising a collection of, for example, video, audio, teletext, closed-captioning and "other data" services. According to the disclosed scheme, a plurality of subframe data streams are generated, each having a transport layer region and a program data region. These subframe data streams are then multiplexed together into superframes having a transport layer region and a subframe data region.
A decoder within a subscriber's home or at any other receiving terminal separates the multiplexed data stream into the various services. One such service is conventional television video images. In order to efficiently transfer video images, redundant information in the image is preferably removed, a process referred to as video compression. A number of video compression techniques are proposed and some techniques have been adopted, for example, International Standards Organization, ISO-11172 and 13818, generally referred to MPEG standards (including MPEG1 and MPEG2) where MPEG refers to Moving Picture Expert Group. Several manufacturers have developed integrated circuits to decompress MPEG 1 and MPEG2 compressed video data, for example Thompson-CSF, C-Cube and LSI Logic Corporation have all developed such decompression integrated circuits.
A standard analog NTSC composite video signal is already a compressed format. Diagonal luminance resolution has been given up in order to accommodate the color subcarrier within the luminance bandwidth. Unfortunately, the existence of the color subcarrier decreases the correlation between adjacent samples and adjacent frames, making it difficult to apply further stages of compression. For this reason, efficient compression algorithms are applied, not to the NTSC signal, but to the original separate components: brightness Y, first color difference, and second color difference or Y, U, V (luminarice and color differences).
Video compression is based on eliminating redundancy from the signal. There are two principal types of redundancy in video signals: psychovisual redundancy and mathematical redundancy. Efficient transmission systems attempt to eliminate both types of redundancy.
Psychovisual redundancy in the signal occurs as a result of transmitting information which the human eye and brain does not use, and cannot interpret. The most obvious example is the excess chrominance bandwidth in R,G,B signals. When a video signal is represented as R,G,B components, none of these components can be reduced in bandwidth without perceptible decrease in picture quality. However, when a full bandwidth luminarice signal is present, the eye cannot detect significant bandwidth reduction of the color information. NTSC makes use of this human psychovisual characteristic by converting the original R,G,B signal to Y,U,V components through a linear matrix. The U,V components can then be reduced in bandwidth to 25% of the luminance component without a loss of apparent picture quality. However, NTSC does not fully exploit color difference redundancy. It reduces bandwidth only in the horizontal dimension, whereas the eye is equally insensitive to chrominance detail in the vertical dimension. Other forms of psychovisual redundancy include: luminance detail on fast-moving objects, luminance diagonal resolution, and luminarice detail close to high contrast ratio transitions (edge masking). Elimination of psychovisual redundancy proceeds by transforming the signal into a domain in which the redundant information can be isolated and discarded.
Mathematical redundancy occurs when any sample of the signal has a nonzero correlation coefficient with any other sample of the same signal. This implies that some underlying information is represented more than once, and can be eliminated leading to data compression. In a minimally sampled television luminance signal of a typical image, adjacent samples are normally 90 to 95% correlated. Adjacent frames are 100% correlated in stationary areas, and more than 90% correlated on average.
In video compression, redundant information in a moving image is removed on a pixel-to-pixel basis, a line-to-line basis and a frame-to-frame basis. The compressed moving video image is transferred efficiently as one part of the services in the multiplexed data stream.
In present day equipment, moving video images are typically encoded in standard signal types such as analog NSTC composite video signals and PAL signals. These signals include periodic repetition of frames of information, each frame including periodic repetition of lines of information (the lines in a frame being organized into first and second fields), and each line including a synchronization information portion and an active video portion. A first subset of the lines of a frame is used for vertical synchronization. A second subset of the lines of a frame is used to transfer ancillary data, such as closed captioning for the deaf on line 21 among the many types of data, during a vertical blanking interval (hereinafter VBI), the data transferred during the vertical blanking interval being referred to as VBI data. The remaining lines, constituting a third subset of the lines of a frame, contain the video image. It is this video image which is most capable of being compressed in the standard video compression techniques.
In video compression techniques, the ancillary data or VBI data is removed from the image before compression. Thus, users who have come to rely on the ancillary VBI data transmitted during the vertical blanking interval of, for example, an NTSC signal, will be unable to enjoy these services when receiving compressed moving video image services transmitted through the multiplex data stream.